Exchange-biased Py/CoO vortex structures: Magnetization reversal, cooling-field dependence, and training

Abstract

In this paper, the magnetization reversal of exchange-biased Py/CoO vortex structures was investigated. The vortex structures were prepared by film deposition onto closely packed arrays of silica particles. By zero field cooling below the N\'eel temperature of the antiferromagnetic CoO, a so called circular exchange bias can be imprinted. By completing the same cooling process in an applied magnetic field, either a displaced vortex or even a $c$ state can be stabilized, which depends strongly on the CoO layer thickness. A strong athermal training effect mainly on the descending branch after the second loop is observed, which is accompanied by a strong reduction in loop shift, indicating a rearrangement of the CoO spin configuration during reversal. By analyzing the nucleation and annihilation field of the exchange-biased loops, a strong asymmetry is observed, which hints to a magnetization reversal via formation of a distorted viscous vortex. This asymmetry gets more pronounced with increasing CoO layer thickness. Furthermore, we investigated the blocking temperature distribution of the Py/CoO sample series. Below the mean blocking temperature, an increased coercive field is observed when the vortex is cooled in a field larger than the annihilation field compared to zero field cooling, revealing the competition between the vortex state formation and the alignment of spins along the cooling-field direction.